The noise power ratio (NPR) of a signal processing element is a measure of its ability to pass a noise loaded input spectrum, shaped to have a notch, without contamination of the notch. This ratio, which is useful in expressing the dynamic range of the signal processing element, is determined by applying a notched input noise spectrum to the element and measuring the contamination in the output notch. Specifically, the depth of the notch in the processed spectrum is examined to determine the noise power ratio of the element.
The above testing method has proven generally effective for estimating the noise power ratio of various signal processing elements and circuits. However, in prior art NPR testers, the input noise spectrum is generated by hardware modules which must be physically plugged into the tester. Each of these modules produces a different input noise spectrum, and therefore to change the spectrum applied to the unit, the operator must physically remove the module from the tester. Moreover, with such a system the operator cannot individually tailor the input spectrum to a specific unit under test. Further, prior art noise power ratio testers are incapable of simultaneously handling both analog and digital signals, and thus systems having one type of signal input and another type of output cannot be effectively tested.
There is therefore a need for an improved method and apparatus for determining the noise power ratio of a unit under test which obviates such plug-in modules for producing the input noise spectrum, and which can accommodate both analog and digital signals.